This invention relates to a drum brake device. More specifically, it relates to a drum brake device for a multi-wheel vehicle, such as an automobile, a forklift, and the like, that functions as a leading-trailing (LT) type when the service brake is applied, and as a duo-servo (DS) type when the parking brake is applied. This type of brake is called a dual mode drum brake.
FIG. 13 is a plan view of the typical rear chassis of the vehicle. The drum brake devices a, b are located on the right and left ends of the axle housing c. The back plates d, d of the drum brake devices a, b are mounted on each end of the axle housing c. The brake drums e, e are located to be exposed at each wheel side and to cover the two drum brake devices a, b. Each drum brake device includes a fluid operated service brake mechanism, a mechanical parking brake mechanism (neither of them shown in the diagram), and the parking brake cables f, g, both of which extend toward the front side of the vehicle.
A dual mode drum brake device has been disclosed in Australian Patent Number AU-B1-53 491/79 and U.S. Pat. No. 5,275,260.
The publicly disclosed drum brake device can be explained with FIG. 14. The upper and lower ends of the pair of brake shoes k, l are contacting the fluid pressure cylinder i and the anchor block j on the back plate d.
The pivot lever n is pivotally provided at the central region of the brake shoe k on the left side of the diagram, with the axis pin m as the fulcrum, while the brake lever o is pivotally provided at the lower portion of the brake shoe l on the right side of the diagram, with the axis pin p as the fulcrum. The parking brake cable f is connected to the free end of the brake lever o.
The first strut q is provided between the upper ends of the brake shoes k, l, and the second strut r is provided between the lower ends of the brake shoes k, l. The right end of the first strut q engages with the upper part of the pivot lever n and brake shoe k, and the left end of the first strut q engages with the brake shoe l. The right end of the second strut r engages with the lower part of the pivot lever n, and the left end of the second strut r engages with the brake lever o. The frictional force caused at the brake shoes k, l, when the parking brake in operation, is transferable between the two brake shoes k, l via the first strut q.
This type of drum brake device has its characteristics that the brake shoe k on the right side of the diagram, when the service brake is applied, operates as a leading shoe and the brake shoe l on the left side of the diagram operates as a trailing shoe, when the brake drum rotates in the direction of the arrow R with moving the vehicle forward.
Comparing the lining wear of the two brake shoes shows a major difference between them. Since the self-servo effect occurs only on a leading shoe, the brake shoe k with the pivot lever n (which is a leading shoe) will be worn about twice more than the brake shoe l (which is a trailing shoe).
The drum brake device as described above needs the following areas of improvement:
The return position of the brake lever is fixed. The pivot lever n is located at the leading shoe side, which is worn the most. The clearance .delta..sub.2 between the pivot lever n and the second strut r increases as the lining is worn. The more the clearance .delta..sub.2 increases, the longer the stroke to effectuate the parking brake becomes. More clearance requiring the longer stroke not only gives disconcerting feeling to the driver when stepping on the brake pedal but also creates a risk of delaying the parking brake effect in an emergency situation.
Additionally, the clearance is prone to cause unintended contact between the other components, especially when a size of the brake device is smaller, thereby causing a greater difficulty in laying out the brake lever.
The parking brake cable g connected to the muffler h side of the drum brake device b, as shown in FIG. 13, must be designed to be installed in the space among the leaf spring s, the shock absorber t, and the muffler h in due consideration of the bounding and rebounding motions of the axle housing c. This makes the layout of the installation of the parking brake cable g more complex.